Method and apparatus for detecting the endpoint in chemical-mechanical polishing of semiconductor wafers

ABSTRACT

A method and apparatus for detecting the endpoint of CMP processing on semiconductor wafer in which a lower layer of material with a first reflectivity is positioned under an upper layer of material with a second reflectivity. Initially an endpoint site is selected on the wafer in a critical area where a boundary between the upper and lower layers defines the desired endpoint of the CMP process. The critical area on the wafer is generally determined by analyzing in the circuit design and the polishing characteristics of previously polished test wafers to denote the last points on the wafer from which the upper layer is desirably removed by CMP processing. After an endpoint site is selected, a light beam impinges the polished surface of the wafer and reflects off of the surface of the wafer to a photo-sensor. The photosensor senses the actual intensity of the reflected light beam. The actual intensity of the reflected light beam is compared with an expected intensity to determine whether the upper layer has been adequately removed from the endpoint site. The actual intensity is preferably compared with an expected intensity for light reflected from one of the upper or lower layers, and the endpoint is preferably detected when the actual intensity of the reflected light beam is either substantially the same as the expected intensity for light reflected from the lower layer or substantially different from the expected intensity for light reflected from the upper layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.08/914,570, filed Aug. 19, 1997, U.S. Pat. No. 5,910,846, which is acontinuation of U.S. patent application Ser. No. 08/650,087 filed May16, 1996, U.S. Pat. No. 5,663,797, issued Sep. 2, 1997.

TECHNICAL FIELD

The present invention relates to a method and apparatus for detectingthe endpoint in chemical-mechanical polishing of semiconductor wafers;more particularly, the present invention detects the endpoint atcritical areas on the wafer where an upper layer of material is noteasily removed from the wafer.

BACKGROUND OF THE INVENTION

Chemical-mechanical polishing ("CMP") processes remove material from thesurface of the wafer in the production of ultra-high density integratedcircuits. In a typical CMP process, a wafer is pressed against apolishing pad in the presence of a slurry under controlled chemical,pressure, velocity, and temperature conditions. The slurry solutiongenerally contains small, abrasive particles that abrade the surface ofthe wafer, and chemicals that etch and/or oxidize the surface of thewafer. The polishing pad is generally a planar pad made from acontinuous phase matrix material such as polyurethane. Thus, when thepad and/or the wafer moves with respect to the other, material isremoved from the surface of the wafer by the abrasive particles(mechanical removal) and by the chemicals (chemical removal) in theslurry.

FIG. 1 schematically illustrates the conventional CMP machine 10 with aplaten 20, a wafer carrier 30, a polishing pad 40, and a slurry 44 onthe polishing pad. An under-pad 25 is typically attached to the uppersurface 22 of the platen 20, and the polishing pad 40 is positioned onthe under-pad 25. In conventional CMP machines, a drive assembly 26rotates the platen 20 as indicated by arrow A. In other existing CMPmachines, the drive assembly 26 reciprocates the platen 20 back andforth as indicated by arrow B. The motion of the platen 20 is impartedto the pad 40 through the under-pad 25 because the polishing pad 40frictionally engages the under-pad 25. The wafer carrier 30 has a lowersurface 32 to which a wafer 12 may be attached, or the wafer 12 may beattached to a resilient pad 34 positioned between the wafer 12 and thelower surface 32. The wafer carrier 30 may be a weighted, free floatingwafer carrier, but an actuator assembly 36 is preferably attached to thewafer carrier 30 to impart axial and rotational motion, as indicated byarrows C and D, respectively.

In the operation of the conventional CMP machine 10, the wafer 12 facesdownward against the polishing pad 40, and then the platen 20 and thewafer carrier 30 move relative to one another. As the face of the wafer12 moves across the planarizing surface 42 of the polishing pad 40, thepolishing pad 40 and the slurry 44 remove material from the wafer 12.CMP processes typically remove either conductive materials or insulativematerials from the surface of the wafer to produce a flat, uniformsurface upon which additional layers of devices may be fabricated.

When a conductive layer is polished from a wafer, the CMP processes mustaccurately stop polishing the wafer at a desired endpoint. Conductivelayers are typically deposited over insulative layers to fill vias ortrenches in the insulative layer and form electrical interconnectsbetween device features on the wafer. To electrically isolate theinterconnects from one another, it desirable to stop the CMP processbelow the top of the insulative layer and above the bottom of theconductive material in the vias and trenches. If the CMP process isstopped before the desired endpoint ("under-polishing"), then theinterconnects will not be electrically isolated from one another andshorting will occur in the circuit. Conversely, if the CMP process isstopped after the desired endpoint ("over-polishing"), theninterconnects may be completely removed from the wafer. Therefore, toavoid serious defects in a wafer, it is highly desirable to stop the CMPprocess at the desired endpoint.

U.S. Pat. No. 5,433,651 to Lustig et al. discloses an apparatus and amethod for determining the endpoint of a CMP process in which a laserbeam passes through a window in the polishing pad and impinges upon thepolished surface of the wafer. The laser beam scans across the surfaceof the wafer, and a photosensor senses the intensity of the beam thatreflects from the wafer. Conductive materials, such as aluminum, have areflectivity index of approximately 90%, while insulative materials,such as boro-phosphate silicon glass ("BPSG"), have a reflectivity indexof approximately 35%. At the endpoint of the CMP process, therefore, theintensity of the reflected beam alternates between that of theconductive material and the insulative material as the laser beam scansacross the wafer. The Lustig et al. patent discloses that the endpointof the CMP process is detected when the intensity of the reflected beamchanges from that of the conductive material to the average intensity ofthe conductive and insulative materials.

One problem with the method of determining the endpoint of the CMPprocess disclosed in the Lustig et al. patent is that it may notaccurately detect the endpoint on wafers that have small "criticalareas." The critical areas are typically depressions on the surface ofthe wafer that are the last point on the wafer from which the conductivematerial is removed by CMP processing. The location and size of thecritical areas is a function of the circuit design and the uniformity ofthe polishing rate across the surface of the wafer. As a result, thecritical areas vary from one type of die to another, and they typicallyoccupy a minuscule portion of the wafer surface. The method disclosed inthe Lustig et al. patent may not detect the status of the CMP process atmany critical areas on the wafer because the critical areas occupy sucha small percentage of the wafer's surface that the few reflectivesignals generated by the critical areas do not statistically impact theoverall average reflectivity of the substantially larger number ofreflective signals from the interconnects. Thus, even if the Lustig etal. patent recognized the problem of critical areas, it may notaccurately detect the endpoint of the CMP process at critical areas onthe wafer.

In light of the problems with detecting the endpoint of the CMP processat critical areas on the wafer, it would be desirably to develop amethod that quickly and accurately detects the endpoint of CMPprocessing at predetermined critical areas on a semiconductor wafer.

SUMMARY OF THE INVENTION

The inventive method and apparatus detects the endpoint of CMPprocessing on semiconductor wafers in which a lower layer of materialwith a first reflectivity is positioned under an upper layer of materialwith a second reflectivity. Initially, an endpoint site is selected onthe wafer in a critical area where a boundary between the upper andlower layers defines the desired endpoint of the CMP process. Thecritical area on the wafer is determined by analyzing the circuit designon the dies of the wafer and the polishing characteristics of previouslypolished test wafers, and preferably denoting the last points on thewafer from which the upper layer is desirably removed by CMP processing.After an endpoint site is selected, a light beam impinges the polishedsurface of the wafer and reflects off of the surface of the wafer to aphoto-sensor. The photosensor senses the actual intensity of thereflected light beam. The actual intensity of the reflected light beamis compared with an expected intensity to determine whether the upperlayer has been removed from the wafer at the endpoint site. The actualintensity is preferably compared with an expected intensity for lightreflected from one of the upper or lower layers, and the endpoint ispreferably detected when the actual intensity of the reflected lightbeam is either substantially the same as the expected intensity forlight reflected from the lower layer or substantially different from theexpected intensity for light reflected from the upper layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a CMP machine inaccordance with the prior art.

FIG. 2 is a partial schematic cross-sectional view of a wafer upon whichthe endpoint detection method of the invention operates.

FIG. 3 is a flowchart of an endpoint detection method in accordance withthe invention.

FIG. 4 is a partial schematic cross-sectional view of the wafer of FIG.2 at one point of the endpoint detection method in accordance with theinvention.

FIG. 5 is a partial schematic cross-sectional view of the wafer of FIG.2 at another point of the endpoint detection method in accordance withthe invention.

FIG. 6 is a partial schematic cross-sectional view of another wafer uponwhich the method of the invention operates.

FIG. 7 is a schematic diagram of an endpoint detector in accordance withthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a method and apparatus that detects theendpoint of CMP processing at critical areas on the surface of asemiconductor wafer. An important aspect of the invention is toselectively impinge a light beam at predetermined endpoint sites on thewafer and measure the intensity of the reflected light. Anotherimportant aspect of the invention is to select the endpoint sites atcritical areas on the wafer that are typically the last points on thewafer from which material of an upper layer is desirably removed by CMPprocessing. By selectively measuring the intensity of the reflectedlight beam at endpoint sites in critical areas of the wafer, the methodof the invention ensures that the CMP process removes the requisiteamount of material from even the most problematic areas on the wafer.Therefore, the endpoint detection method of the invention reduces thenumber of defects on a wafer caused by over-polishing or under-polishingthe wafer at the critical areas. FIGS. 2-5, in which like referencenumbers referred to like parts throughout the various figures,illustrate the method of the invention and a semiconductor wafer uponwhich the method of the invention operates.

FIG. 2 illustrates a wafer 12 before it has been polished by a CMPmachine 10, as discussed above with respect to FIG. 1. The wafer 12 is atypical wafer upon which the method of the invention operates to detectthe endpoint of the CMP polishing process, but it will be appreciatedthat the endpointing method of the invention is not limited to anyspecific wafer design or circuit design on a wafer. The wafer 12 has asubstrate 50, a number of large devices 52 formed on the substrate 50,and a small device 54 formed on the substrate 50 between the largedevices 52. The large and small devices 52 and 54 are film stacks thatform very small electrical components of the integrated circuits on anindividual die of the wafer 12. Depending upon the number of layers ineach film stack, the top surfaces of the large and small devices 52 and54 are generally spaced away from the upper surface 51 of the substrate50 by different axial distances (as measured with respect to thevertical axis indicated by line Z--Z).

A lower layer 60 is deposited over the substrate 50 and the devices 52and 54 to provide a platform above the devices 52 and 54 upon whichadditional devices may be formed. The lower layer 60 generally followsthe topography of the substrate 50 and devices 52 and 54 so that thelower layer 60 has high points 62 positioned over the large devices 52and depressions 64 positioned between the large and small devices 52 and54. A number of vias 66 etched into the lower layer 60 provide access tothe devices 52 and 54 so that other devices (not shown) fabricated ontop of the lower layer 60 may be electrically coupled to the devices 52and 54. The lower layer 60 is generally made from an insulative materialsuch as BPSG, silicon dioxide, silicon nitride, or other knowninsulative materials. The materials from which the lower layer 60 ismade have a first reflectivity, which in the case of BPSG isapproximately 0.35.

An upper layer 70 is deposited over the lower layer 60 and into the vias66 to form interconnects 76 with the device features 52 and 54. Theupper layer 70 is generally made from a conductive material that has asecond reflectivity, which in the case of aluminum is approximately0.90. The upper layer 70 generally follows the contour of the lowerlayer 60 such that the bottom of the upper layer 70 fills thedepressions 64 in the lower layer 60.

The depressions 64 define critical areas 92 on the wafer where thematerial of the upper layer 70 is typically the last portion of theupper layer 70 that is desirably removed from the wafer by CMPprocessing. All of the conductive material of the upper layer 70 in thedepressions 64 must be removed from the wafer to avoid forming unwantedconductive lines across the top of the wafer 12 that may short onedevice to another and destroy the operation of the circuit. Thus, thecritical areas 92 on the wafer 12 are defined by the last desirableportions of the upper layer 70 that are removed from the surface of thewafer 12 by CMP processing.

The location and size of the critical areas 92 is a function of thespecific circuit designs, previous planarizing steps in the processflow, and the polishing characteristics at the location of each die onthe face of the wafer. The circuit designs often result in depressionsor other areas on the surface of the wafer that are either lower thanother areas on the wafer or are hard to reach with a polishing pad. InFIG. 2, for example, the bottoms of the depressions 64 are the lowestpoints on the surface of the lower layer 60. The depressions 64 are alsohard to reach with a polishing pad because the top surfaces 62 of thelower layer 60 surround the depressions 64 and support the surface ofthe polishing (not shown) pad over the depressions 64. Moreover, sincethe polishing rate may vary across the face of the wafer, one die in thewafer may have a different polishing rate than another die. Therefore,to determine the critical areas 92 for a specific design, it isimportant to analyze the circuit design and the polishingcharacteristics to determine the areas on the wafer where the lastdesirable portion of the upper layer 70 is typically removed from thewafer by CMP processing.

FIGS. 2 and 3 together illustrate the operation of the method of theinvention. The first step of the method 110 is to select an endpointsite 90 at a critical area 92 in the wafer 12. The endpoint site 90 ispreferably located just under the lowest point of the critical area 92so that all of the conductive material of the upper layer 70 will beremoved from the wafer 12 when the endpoint of the CMP process isobtained. The critical areas 92 and the endpoint sites 90 are determinedby analyzing the specific circuit structure of the dies on the wafer toestimate the most likely locations of the critical areas, and thenpolishing several test wafers with the specific circuit structure usingthe polishing parameters of the selected CMP process. After the testwafers are polished, a laser scans the estimated locations of criticalareas to empirically determine the actual locations of critical areas 92on the wafer 12. Once the location of the critical areas and endpointsites is ascertained, several endpoint sites on each wafer arepreferably selected to ensure the desired endpoint of the CMP processobtained across the whose surface of the wafer.

In a preferred embodiment, the second step 120 of the method of theinvention is to align a laser beam with an endpoint site 90 on the wafer12. In general, a laser 80 with an emitter 82 directs a laser beam 84 toimpinge the surface of the wafer 12, and a reflected beam 86 reflectsfrom the surface of the wafer 12 to a photosensor 88. The photosensor 88senses the intensity of the reflective beam 86, which depends uponwhether the laser beam 84 impinges the upper layer 70 or the lower layer60. The laser beam 84 may be aligned with the endpoint sites 90 bymoving either the laser emitter 82 or the wafer 12 to pass the laserbeam 84 over the face of the wafer 12. In one embodiment, the laser beam84 is aligned with the endpoint site 90 by noting the X,Y coordinates ofthe endpoint site 90 with respect to a reference point on the surface ofthe wafer, and then moving the wafer to position the X,Y coordinate ofthe endpoint site in the path of the laser beam 84. In a preferredembodiment, the laser beam is aligned with the endpoint 90 bypositioning the wafer 12 using a pattern recognition spectrophotometer,such as the NovaScan 210 manufactured by NOVA Corporation of Israel. Intypical pattern recognition spectrophotometers, a laser passes over thecircuit to record the pattern of the circuit. The recorded pattern ofthe circuit is correlated with an actual diagram of the circuitprogrammed into a pattern recognition module to determine the positionof the laser beam 84 with respect to the components of the circuit onthe wafer. The laser beam 84 is preferably aligned with the endpointsite 90 to within approximately less than 1.0 μm of the center of theendpoint site. Other suitable spectrophotometers that can appropriatelyalign a laser beam with the endpoint site 90 include spectrophotometersmanufactured by Tencor Corporation of California and ThermawaveCorporation of California.

After the laser beam 84 is aligned with the endpoint site 90, the nextstep 130 of the method of the invention is to impinge the laser beam 84against the wafer 12. In one embodiment, the surface of the wafer ispositioned in a clean air environment, and the laser beam 84 passesthrough the air and directly impinges the surface of the wafer 12. Oneproblem with positioning the surface of the wafer in air, however, isthat the surface of the wafer dries out and must be cleaned and wettedto avoid scratches before the wafer may be repolished. Accordingly, in apreferred embodiment, the face of the wafer 12 is placed in waterimmediately after being removed from the CMP machine, and the laser beam84 passes through the water to impinge the surface of the wafer. Byplacing the face of the wafer 12 in water, the wafer may be repolishedwithout cleaning or wetting the wafer. One suitable spectrophotometerthat impinges a laser beam against a wafer while the face of the waferis under water is the NovaScan 210 manufactured by NOVA Corporation.

The next step of the invention 140 is to sense the actual intensity ofthe reflected light beam 86. A photosensor 88 positioned in the path ofthe reflected light beam 86 senses the intensity of the reflected lightbeam as a percentage of the intensity of the original laser beam 84.Different wavelengths of light may be used such that the intensity ofthe reflected light may be measured at different wavelengths to get amore reliable signature of the material. The intensity of the reflectedlight beam 86 will depend upon the type of material at the surface ofthe wafer impinged by the laser beam 84. If the CMP process is stoppedsuch that the polished surface of the wafer is vertically spaced awayfrom the surface 51 of the substrate 50 at a height defined by the lineA--A, the intensity of the reflected light beam 86 is theoreticallyequal to the product of the intensity of the laser beam 84 and thereflectivity index of the upper layer 70. In the case of conductivematerials, such as aluminum or tungsten, the reflectivity index isapproximately 0.9 such that the intensity of the reflected beam 86 isapproximately 90% of the original intensity of the laser beam 84.Similarly, if the CMP process is stopped such that the polished surfaceof the wafer is vertically spaced away from the surface 51 of thesubstrate 50 by a height defined by line B--B, the intensity of thereflected light beam 86 is theoretically equal to the product of theintensity of the laser beam 84 and the reflectivity index of the lowerlayer 60. In the case of BPSG, which has a reflectivity index ofapproximately 0.35, the intensity of the reflected beam at a heightindicated by line B--B is approximately 35% of the original intensity ofthe laser beam 84. Once again, the NovaScan 210 spectrophotometersuitably detects a change in intensity of the reflected beam 86. Sincethe endpoint 90 preferably lies in a plane that is just below thelowermost point of the depression 64 (as indicated by line B--B), theexpected intensity of the reflected beam 86 at the desired endpointchanges from the theoretical intensity of light reflected from the upperlayer 70 to the theoretical intensity of light reflected from the lowerlayer 60.

The final step 150 of the method is to compare the actual intensity ofthe reflected light beam 86 with an expected intensity at the endpointof the CMP process. The expected intensity of the reflected beam at theendpoint of the CMP process is the theoretical intensity of lightreflected from either the upper layer 70 or the lower layer 60. In thecase where the expected intensity is the theoretical value of lightreflected from the upper layer 70, the endpoint is reached when theactual intensity of the reflected beam 86 is substantially differentthan that of the theoretical intensity of light reflected from the upperlayer 70. In the case where the expected intensity is equal to theintensity of light reflected from the lower layer 60, the endpoint isindicated when the actual intensity of the reflected light beam 86 issubstantially equal to the theoretical intensity of light reflected fromthe lower layer 60.

The advantages of the method of the invention are best illustrated inFIGS. 4 and 5. Referring to FIG. 4, the surface of the wafer 12 isunder-polished at the level indicated by line A--A because smallportions of the upper layer 70 in the depressions 64 remain on thepolished surface of the wafer. If the endpoint of the wafer 12 wasscanned by a laser in accordance with the Lustig et al. patent, thepresence of the critical areas may not be detected because the criticalareas occupy only a small percentage of the surface area on the waferand can generate only few high-reflection signals. Since the Lustig etal. patent treats the high-reflection signals from the critical areas ascoming from just another interconnect on the wafer, the relatively fewreflective signals from the critical areas will not statistically impactthe overall average of the much larger number of reflective signals fromthe interconnects. Therefore, because the scanning device disclosed inthe Lustig et al. patent does not specifically detect the presence ofthe conductive material at the critical areas, it may falsely indicatethat the endpoint of the CMP process has been reached.

The primary advantage of the method of the present invention is that itensures that the material of the upper layer 70 in the depressions 64 isdetected by the laser 80. Since the laser beam 84 is aligned with anendpoint site 90 positioned below the bottom of a critical area 92, themethod of the invention ensures that the reflectivity at critical areason the wafer is adequately measured. Referring to FIGS. 4 and 5together, the method of the present invention 1 indicates that the CMPprocess should further planarize the surface of the wafer from the levelat line A--A to the level at line B--B. Therefore, the present inventionprovides a method that accurately detects the endpoint of the CMPprocess at even the most problematic points on the wafer where the lastportions of an upper layer of material are desirably removed from thewafer.

FIG. 6 illustrates another wafer 14 after it has been polished to alevel A--A. Wafer 14 has a number of devices 58(a)-(c) asymetricallyspaced apart from one another across the substrate 50. The distancebetween the devices 58(a) and 58(b) produces a small depression 64 inthe lower layer 60, while the larger distance between devices 58(b) and58(c) produces a large, deep depression 64. As with the depressions 64illustrated in FIGS. 2 and 4, the upper layer 70 fills the depressions64 and defines another set of critical areas on the wafer 14 from whichthe upper layer 70 is desirably removed. The method of the inventionalso detects the endpoint of wafer 14 in the same manner as wafer 12 ofFIGS. 2 and 4. Thus, the method of the invention may be used to detectthe endpoint on many different wafer structures.

FIG. 7 illustrates an endpoint detector 100 for performing the method ofthe invention. The endpoint detector 100 has a pattern recognitionspectrophotometer 110 for selectively impinging the laser beam 84 atcritical areas on the surface of the wafer 12, and a water track 170 inwhich the wafer 12 positioned. In the pattern recognitionspectrophotometer 110, an actuator 120 connected to a laser 80translates the laser 80 in a plane substantially parallel to the face ofthe wafer 12, as defined by the X-Y axes. The laser 80, which isdescribed above with respect to FIGS. 2 and 3, has an emitter 82 thatdirects the laser beam 84 to the wafer 12, and a photosensor 88 forsensing the reflected beam 86 from the wafer 12. A processor/controller130 is operatively connected to the actuator 120 and to a patternrecognition module 140. Importantly, the pattern recognition module 140has a database containing the circuit pattern of the specific integratedcircuit on the wafer 12 and the locations of the critical areas in thecircuit. The processor/controller 130 receives signals from thephotosensor 88 via line 89, and it compares the signals from thephotosensor 88 with the circuit pattern in the recognition module 140 todetermine the location of the laser beam 84 on the face of the wafer 12.One suitable endpoint detector 100 is the NovaScan 210 with a patternrecognition module in which the database contains a circuit pattern of acircuit on the wafer and the location of endpoint sites on the circuitpattern.

In operation, the wafer 12 is mounted in the water track 170 so that theface of the wafer 12 is positioned under the water surface 172 inalignment with an optical window 174 in the bottom of the wafer track170. The emitter 82 directs the laser beam 84 through the optical window174 against the wafer 12, and the reflected beam 86 passes back throughthe optical window 174 to the photosensor 88. As the actuator 120 movesthe laser 80 across the wafer, the processor/controller 130 correlatesthe signals from the photosensor 88 with the circuit pattern programmedin the pattern recognition module 140 to determine the location of thelaser beam 84 with respect to the pattern of the circuit on the wafer12. The processor/controller 130 then directs the actuator 120 to movelaser beam 84 across the surface of the wafer 12 until the beam 84 isaligned with a critical location. The photosensor 88 senses thereflectivity of the wafer 12 at the critical area, and it sends a signalrepresenting the reflectivity at the critical area to theprocessor/controller 130. As described above with respect to the methodof the invention, the processor/controller 130 compares the actualreflectivity of the material at the critical area with an expectedreflectivity at the endpoint of the CMP process to determine if thewafer is at a desired endpoint.

It will be appreciated that, although specific embodiments of theinvention have been described herein for purposes of illustration,various modifications may be made without departing from the spirit andscope of the invention. Accordingly, the invention is not limited exceptas by the appended claims.

I claim:
 1. A method for detecting an endpoint in chemical-mechanicalpolishing of a semiconductor wafer having a lower layer of material witha first reflectivity and an upper layer with a second reflectivitypositioned over the lower layer, wherein the upper layer is polishedwith a chemical-mechanical polishing process to form a polished surfaceon the wafer, the method comprising:selecting an endpoint site on thewafer defined by a boundary between the upper and lower layers at aselected location on a topography of a circuit of the wafer, theselected location being at a depression in the boundary between theupper and lower layers; impinging a beam of light against the polishedsurface of the wafer, the beam of light being reflected to a lightsensor; sensing an actual intensity of the reflected beam of light; andcomparing the actual intensity of the reflected light beam with anexpected intensity.
 2. The method of claim 1 wherein when the endpointis not detected at the comparing step, the method further comprises thestep of re-polishing the wafer to form a re-polished surface on thewafer and repeating the impinging, sensing, and comparing steps on there-polished surface.
 3. The method of claim 1 wherein the selecting stepcomprises choosing multiple endpoint sites on the wafer and performingthe impinging, sensing, and comparing steps at each endpoint site. 4.The method of claim 1 wherein the selecting step comprises estimatingpoints on the wafer at which portions of the upper layer areconsistently the last parts of the upper layer desirably removed fromthe wafer before the desired endpoint, and scanning the estimated pointson polished test wafers to determine actual points on the wafer at whichportions of the upper layer are consistently the last parts of the upperlayer desirably removed from the wafer before the desired endpoint. 5.The method of claim 1 wherein the comparing step further comprisescomparing the actual intensity of the reflected light beam with anexpected intensity for light reflected from one of the upper and lowerlayers, the endpoint being detected when the actual intensity of thereflected light beam is either substantially the same as the expectedintensity for light reflected from the lower layer or substantiallydifferent from the expected intensity for light reflected from the upperlayer.
 6. The method of claim 1 wherein the method further comprisesaligning a beam of light with the endpoint site.
 7. The method of claim6 wherein the wafer has a number of dies and each die has a circuitpattern, the aligning step comprising scanning the circuit pattern oneach die and positioning the endpoint site in the path of the lightbeam.
 8. The method of claim 1 wherein the upper layer is made from asubstantially reflective material, and the lower layer is made from asubstantially non-reflective material.
 9. The method of claim 8 whereinthe upper layer is made from aluminum.
 10. The method of claim 9 whereinthe upper layer is made from tungsten.
 11. The method of claim 1 whereinthe polished surface on the wafer is disposed in a fluid during theimpinging and sensing steps.
 12. A method of chemical-mechanicalpolishing of a semiconductor wafer, comprising:positioning a waferadjacent to a polishing surface of a polishing pad in the presence of aplanarizing solution, the wafer having a lower layer of material with afirst reflectivity and an upper layer with a second reflectivitypositioned over the lower layer; moving at least one of the wafer andthe polishing pad with respect to the other to impart relative motiontherebetween, the polishing pad and the planarizing solution removingmaterial from the wafer to produce a polished surface on the wafer;selecting an endpoint site on the wafer defined by a boundary betweenthe upper and lower layers at a selected location on a topography of acircuit on the wafer, the selected location being at a depression in theboundary between the upper and lower layers; impinging a beam of lightagainst the polished surface of the wafer, the beam of light beingreflected to a light sensor; sensing an actual intensity of thereflected beam of light; and comparing the actual intensity of thereflected light beam with an expected intensity.
 13. The method of claim12 wherein when the endpoint is not detected at the comparing step, themethod further comprises the steps of:re-positioning the wafer adjacentto the polishing surface of the polishing pad in the presence of theslurry; moving at least one of the wafer and the polishing pad withrespect to the other to impart relative motion therebetween, thepolishing pad and slurry removing material from the wafer to produce are-polished surface on the wafer; and repeating the impinging, sensing,and comparing steps on the re-polished surface.
 14. The method of claim12 wherein the selecting step comprises choosing multiple endpoint siteson the wafer and performing the impinging, sensing, and comparing stepsat each endpoint site.
 15. The method of claim 12 wherein the selectingstep comprises estimating points on the wafer at which portions of theupper layer are consistently the last parts of the upper layer desirablyremoved from the wafer before the desired endpoint, and scanning theestimated points on polished test wafers to determine actual points onthe wafer at which portions of the upper layer are consistently the lastparts of the upper layer desirably removed from the wafer before thedesired endpoint.
 16. The method of claim 12 wherein the comparing stepfurther comprises comparing the actual intensity of the reflected lightbeam with an expected intensity for light reflected from one of theupper and lower layers.
 17. The method of claim 12 wherein the methodfurther comprises aligning the beam of light with the endpoint.
 18. Themethod of claim 12 wherein the upper layer is made from a substantiallyreflective material, and the lower layer is made from a substantiallynon-reflective material.
 19. The method of claim 18 wherein the upperlayer is made from aluminum.
 20. The method of claim 18 wherein theupper layer is made from tungsten.
 21. The method of claim 12 whereinthe polished surface on the wafer is disposed in a fluid during theimpinging and sensing steps.
 22. An endpoint detector for detecting theendpoint in chemical-mechanical polishing of a semiconductor waferhaving a lower layer of material with a first reflectivity and an upperlayer with a second reflectivity positioned over the lower layer,wherein the upper layer is polished with a chemical-mechanical polishingprocess to form a polished surface on the wafer, the detectorcomprising:a pattern recognition module having a database containing acircuit pattern of a circuit on the wafer and a location of an endpointsite on the circuit pattern, the endpoint site being defined by aboundary between the upper and lower layers at a selected location onthe circuit defined by a depression in the boundary between the upperand lower layers; a laser having an emitter that directs a laser beam atthe wafer and a photosensor that senses an intensity of a reflected beamthat reflects from the polished surface of the wafer; an actuatorconnected to one of the laser or the wafer, the actuator moving the oneof laser or the wafer in a plane substantially parallel to the polishedsurface of the wafer so that the laser beam passes over the polishedsurface of the wafer; and a controller operatively coupled to thepattern recognition module, the photosensor, and the one of the laser orthe wafer to which the actuator is connected, the controller correlatingsignals from the photosensor with the circuit pattern in the patternrecognition module to determine where the laser beam impinges the waferand the controller directing the actuator to move the one of the laserbeam or the wafer until the laser beam is aligned with the endpointsite, wherein the photosensor senses the reflectivity of material on thepolished surface at the endpoint site and the controller compares thesensed reflectivity at the endpoint site with an expected reflectivityof either the upper or lower layers.
 23. The endpoint detector of claim22, further comprising a water track having an optical window positionedto face the laser, the track being filled with water and the polishedsurface of the wafer being submerged in the water in alignment with theoptical window, wherein the emitter directs the laser beam through theoptical window and the water to impinge upon the endpoint site on thepolished surface of the wafer.
 24. A method for detecting an endpoint inchemical-mechanical polishing of a semiconductor wafer having a lowerlayer of material with a first reflectivity and an upper layer with asecond reflectivity positioned over the lower layer, wherein the upperlayer is polished with a chemical-mechanical polishing process to form apolished surface on the wafer, the method comprising:selecting anendpoint site on the wafer defined by a boundary between the upper andlower layers at a selected location on a topography of a circuit of thewafer, the selected location being at a critical site between the upperand lower layers that is typically a last location on the wafer fromwhich the upper layer is desirably removed; reflecting a beam of lightfrom the polished surface of the wafer to a light sensor; sensing anactual intensity of the reflected beam of light; and comparing theactual intensity of the reflected light beam with an expected intensity.25. A method of chemical-mechanical polishing of a semiconductor wafer,comprising:positioning a wafer adjacent to a polishing surface of apolishing pad in the presence of a planarizing solution, the waferhaving a lower layer of material with a first reflectivity and an upperlayer with a second reflectivity positioned over the lower layer; movingat least one of the wafer and the polishing pad with respect to theother to impart relative motion therebetween, the polishing pad and theplanarizing solution removing material from the wafer to produce apolished surface on the wafer; selecting an endpoint site on the waferdefined by a boundary between the upper and lower layers at a selectedlocation on a topography of a circuit on the wafer, the selectedlocation being at a site between the upper and lower layers that istypically a last location on the wafer from which the upper layer isdesirably removed; reflecting a beam of light from the polished surfaceof the wafer to a light sensor; sensing an actual intensity of thereflected beam of light; and comparing the actual intensity of thereflected light beam with an expected intensity.
 26. A method fordetecting an endpoint in chemical-mechanical polishing of asemiconductor wafer having a lower layer of material with a firstreflectivity and an upper layer with a second reflectivity positionedover the lower layer, wherein the upper layer is polished with achemical-mechanical polishing process to form a polished surface on thewafer, the method comprising:selecting an endpoint site on the waferdefined by a boundary between the upper and lower layers at a selectedlocation on a topography of a circuit of the wafer, the selectedlocation being defined by a low point of the upper layer that is to beremoved from the wafer; reflecting a beam of light from the polishedsurface of the wafer to a light sensor; sensing an actual intensity ofthe reflected beam of light; and comparing the actual intensity of thereflected light beam with an expected intensity.
 27. A method ofchemical-mechanical polishing of a semiconductor wafer,comprising:positioning a wafer adjacent to a polishing surface of apolishing pad in the presence of a planarizing solution, the waferhaving a lower layer of material with a first reflectivity and an upperlayer with a second reflectivity positioned over the lower layer; movingat least one of the wafer and the polishing pad with respect to theother to impart relative motion therebetween, the polishing pad and theplanarizing solution removing material from the wafer to produce apolished surface on the wafer; selecting an endpoint site on the waferdefined by a boundary between the upper and lower layers at a selectedlocation on a topography of a circuit on the wafer, the selectedlocation being defined by a low point of the upper layer that is to beremoved from the wafer; reflecting a beam of light from the polishedsurface of the wafer to a light sensor; sensing an actual intensity ofthe reflected beam of light; and comparing the actual intensity of thereflected light beam with an expected intensity.